High chromium alloys of improved workability

ABSTRACT

WORKABILITY AND TORSION DUCTILITY OF HIGH CHROMIUMNICKEL (E.G., 50% CHROMIUM) AND CHROMIUM-NICKEL-IRON ALLOYS IMPROVED WHEN THE ALLOYS CONTAIN AN EFFECTIVE AMOUNT OF AN EUTECTIC FORMING ELEMENT FROM THE GROUP ZIRCONIUM, HAFNIUM, YTTRIUM AND CERIUM.

Dec. 14, 1971 TAYLOR ETAL 3,627,511

HIGH CIIROMIUM ALLOYS OF IMPROVED WORK/\BILI'IY Filed Feb. 7, 1969 INV/QN'IURS 59090 [45202 My E m/p dq/vc's yeey K bpvm AffCLQNC) 3,627,511 HIGH CHROMIUM ALLOYS OF IMPROVED WORKABILITY Brian Taylor, Birmingham, and Philip James Parry, Solihull, England, assignors to The International Nickel Company, Inc., New York, N.Y.

Filed Feb. 7, 1969, Ser. No. 797,515 Claims priority, application Great Britain, Feb. 8, 1968, 6,290/ 68 Int. Cl. C22c 39/20, 39/54 U.S. Cl. 75-122 ll Claims ABSTRACT OF THE DISCLOSURE Workability and torsion ductility of high chromiumnickel (e.g., 50% chromium) and chromium-nickel-iron alloys improved when the alloys contain an effective amount of an eutectic forming element from the group zirconium, hafnium, yttrium and cerium.

As is well known to those skilled in the art, it has long been recognized that high chromium alloys containing upwards of 30% to 75% chromium offer-a most desirable combination of strength and resistance to various corrosive media. For example, alloys containing, say, 40% or 50% and up to 70% chromium, the balance being nickel, with or without iron, possess good strength and exhibit particularly outstanding resistance to the degradation effects of fuel ash at elevated temperatures. For such reasons alloys of this type can be used to advantage in furnaces, e.g., as extruded thin walled tubes, and also as extruded welding wire for welding articles or components of furnaces and similar equipment.

Equally well known, however, is the fact that such alloys are exceptionally difficult to work, though naturally their workability characteristics differ depending upon the percentage of chromium, being increasingly more diflicult to work as the chromium content is increased. Paradoxically, the situation has been one in which the very constituent, chromium, largely responsible for the strength and corrosion resistant characteristics of such alloys is also the principal cause of the very tenuous commercial problem-poor workability. The severity of the problem is highlighted, for example, by reference to thebinary chromium-nickel alloys in which even components of simple design are nearly always produced as castings since the alloys, being virtually unworkable, are extremely brittle. Suflice to say, this is quite unfortunate in view of the fact that the ultimate product produced is limited in size, shape, etc., as a result of the casting process.

It is true that, in general, workability is improved once the cast structure of an ingot has been initially broken down by some technique such as extrusion whereby the alloys are then placed in the wrought rather than cast state. However, even in this wrought condition the alloys still manifest low ductility at high temperature as measured by the torsion plastometer and are still anything but amenable to hot-forming into finished articles, particularly at chromium levels of 40% or 50% or more. Too, attempts have been made in the past to increase room temperature ductility by heat treatment, and specifically wrought 50-50 chromium-nickel alloys have been heated to a temperature of about 1200 C. or above and then water-quenched. This has not proved to be an acceptable panacea. Accordingly, the present invention in the main is directed to minimizing or overcoming the torsional ductility problem but without an attendant sacrifice in other beneficial characteristics for which high chromium alloys have long been noted.

ited States Patent ice It has now been discovered that provided the alloy composition is one in which a nickel eutectic is formed the torsion ductility of high chromium alloys in the wrought condition can be strikingly enhanced to such an extent that they are more readily workable to shape. More specifically, it has been found that the presence of certain elements, particularly zirconium, results in a marked improvement in torsional ductility when present as a nickel eutectic as further explained herein.

It is an object of the present invention to provide alloys of high chromium content which offer enhanced torsional ductility in combination with good strength and corrosion resistance.

Other objects and advantages will become apparent from the following description taken in conjunction with the accompanying drawing in which there is depicted a ternary diagram relationship among the constituents chromium, nickel and iron.

Generally speaking and in accordance with the present invention, high chromium alloys contemplated herein contain (weight percent) from 29% to about chromium, up to 46% iron, at least one constituent selected from the group consisting of zirconium, yttrium, hafnium and cerium in an amount at least suflficient to form a nickel eutectic, up to about 1% manganese, up to about 1% silicon and the balance nickel. As will be understood by those skilled in the art, the term balance or balance essentially used herein in referring to the nickel content does not exclude the presence of other elements such as those commonly present as incidental elements, e.g., deoxidizing and cleansing elements, and impurities normally associated therewith, in small amounts which do not adversely affect the basic characteristics of the alloys. In this connection, the impurities include carbon, which normally should not exceed 0.1%, sulfur and phosphorus, which should not usually be allowed to exceed 0.02% each, and nitrogen. Examples of incidental elements are titanium and aluminum which can be present in small amounts, e.g. not above 0.2% each in the subject alloys. However, the total amount of elements other than chromium, nickel, iron and the eutectic forming constituent should not exceed about 2.5%. It is to be understood that the base composition includes in addition to the well known binary nickel-chromium alloys, the ternary chromium-nickel-iron alloys of 30% Cr/60% Ni/ 10% Fe, 50% Cr/30% Ni/20% Fe and 70% Cr/20% Ni/10% Fe, in all of which, as is well known, the content of each of the elements may depart from the nominal value by i-2%.

In carrying the invention into practice, the chromium, nickel and iron contents within the ranges above set forth, must be so correlated that the alloy base is represented by a point in or on the lines bounding the area ABCDEA in the ternary diagram shown in the accompanying drawing. The alloys in the area BCDFB are two-phase (alpha gamma), a fact which explains at least in part the difiiculty in working them, and those in the area ABFEA, though nominally composed only of the single gamma phase, tend in practice to contain some alpha phase. All these alloys are far more difficult to work than, for example, the /20 nickel-chromium alloys that are extensively used as resistance elements.

The most advantageous eutectic-forming element is zirconium, although yttrium, hafnium and to a lesser extent cerium can be used instead. However, care must be exercised regarding the percentage of the eutectic former used. It must be an effective amount as contemplated herein. To explaineach of the elements zirconium, yttrium, hafnium and cerium has a very considerable affinity for such constituents as nitrogen. Nitrogen, though not an element normally mentioned in specifications of chromium-nickel and chromium-nickel-iron alloys is in fact invariably present as an impurity. Typically, alloys of the type in question contain up to 0.2% nitrogen, although up to 0.3% is commonly permitted in certain areas and at times the nitrogen content runs as high as 0.4%. To form the desired eutectic there must be an amount of zirconium or other eutecticforming element in excess of the amount which will combine with the nitrogen to form the corresponding nitride (and this would be applicable with regard to other elements with which the eutectic forming element would combine if present in the alloy). This excess amount is referred to herein as the effective zirconium (or cerium, yttrium or hafnium).

Considering zirconium first, the amount combined as nitride is calculated as 6.5 times the nitrogen content meaning, if 0.1% nitrogen is present there must be at least 0.65% zirconium to counteract the nitrogen effect. Because of a tendency for the uncombined eutectic-forming elements to dissolve in chromium, thus not being available to form the desired eutectic with nickel, it is desirable to ensure the presence of more of such eutectic forming constituent in relation to the chromium content. Accordingly, it is most advantageous that the effective zirconium should be a least 0.2% when the chromium content is 30%, at least 0.35% when the chromium content is 70%, and a proportionate minimum value at intermediate chromium contents. Therefore, at 0.1% nitrogen and 70% chromium the total zirconium should be at least 1%, though the minimum effective amount is 0.35 Naturally, the improvement in ductility increases as the amount of eutectic: formed increases above a trace, but the amount of zirconium or other eutectic-forming elements preferably does not exceed 2% though it may be as high as 4% Similar amounts of effective yttrium, hafnium and cerium are required, but the amount of each of these elements combined as nitride differs. The effective cerium is that in excess of 9 times the nitrogen content, the effective yttrium that in excess of 6 times the nitrogen content and the effective hafnium that in excess of 13 times the nitrogen content.

It is desirable to keep the nitrogen content of the alloys as low as possible, and preferably below 0.1% since zirconium (or other) nitride tends adversely to affect the workability of the alloys, but in practice we find it difficult to reduce it below 0.005% or even 0.01% in alloys with the higher chromium contents. To insure that the nitrogen is kept to a suitably low value the alloys are preferably made by vacuum melting, but they may be made by air melting providing that the surface of the molten metal is covered by a suitable slag and shielded with an inert gas. The nitrogen usually enters the melt from the chromium, and it is preferred to use chromium made by the aluminothermic process and having a low nitrogen content, e.g., about 0.01%.

When the zirconium or other eutectic-forming element is added to a molten chromium-nickel alloy some of it is lost, but suitable adjustments can be readily made. For example, assuming that the element is zirconium, the alloy can be made by melting the chromium and nickel and, at this point, ascertaining the nitrogen content by tapping the melt and determining the residual nitrogen by rapid vacuum or inert-gas fusion techniques. Thereupon, the zirconium is added in an amount based on calculation after the determination of the nitrogen content. When the melting is effected in air, we may add up to three times the calculated zirconium content to the melt in order to produce a desired effective zirconium content. In vacuum melting and casting We add less. Since the presence of the desired eutectic of nickel and zirconium is readily ascertainable under microscopic examination in the as-cast alloy, its existence may readily be ascertained by taking a sample of the melt before casting and preparing a section, and examining it under the nucroscope.

The complete theoretical explanation for the mechanism involved is not fully understood. At any rate, although the invention depends on adding zirconium (or other eutectic forming element above mentioned) in an amount effective to form a eutectic, it is found that this eutectic is destroyed in the course of hot working. It appears that it particularly facilitates the initial conversion of the alloy from the cast stage to the wrought state, and that when he initial structure has been completely broken down the eutectic is replaced by the intermetallic compound Ni Zr with chrornium in solution. Essentially the same applies when one of the other elements is used, the equivalent intermetallic compounds being Ni Ce, Ni Hf and Ni Y For the purpose of giving those skilled in the art a better appreciation of the invention, the following illustrative examples and data are given:

EXAMPLE I An alloy nominally of 35% chromium and 65% nickel was made by first melting nickel pellets in air, deoxidizing the melt with a carbon rod and killing it with an addition of 0.05% silicon. Thermic chromium containing about 0.01% nitrogen was added to the melt which was then covered with a 3 :1 lime/ calcium fluoride slag while using an argon shield to avoid nitrogen pick-up from the atmosphere. The melt was then deoxidized with 0.15% silicon and 0.2% aluminum. A nitrogen analysis at this point gave a nitrogen content of 0.049% whereupon 1% zirconium was added. The melt was cast at 1520 C. into an ingot 4 inches in length and 1.875 inches in diameter, and hammer-forged at a temperature of 1150 C. to bar having a diameter of 0.625 inch. The zirconium content was found to be 0.55 and the nitrogen content to be 0.049%, giving an effective zirconium content of about 0.23%. The forged bar was machined to form a test specimen of 2.375 inch gauge length having a diameter of 0.252 inch and tested at 1000 C. in a torsion plastometer at 46 revolutions per minute. The bar fractured after 63 revolutions.

In marked contrast, a bar of an alloy of similar composition but Without zirconium and worked in exactly the same way gave only 6 revolutions under identical con ditions of test. At 850 C. and at 1050' C. test bars gave 5 and 9.5 revolutions to fracture, respectively.

It should be noted that the above alloy was hammerforged and this point is of particular interest for it has been found that alloys containing up to 50% or 60% chromium, e.g., 40% to 60% chromium, can be so forged provided the eutectic, particularly the zirconium eutectic, is present. This eliminates reliance on extrusion and whatever inherent limitations are associated therewith.

EXAMPLE II The need for an effective amount of zirconium is further illustrated by another alloy which contained 35% chromium and balance nickel. This alloy was produced and processed in the manner as described in Example I except an addition of only 0.2% zirconium was made to the melt. The retained zirconium content was 0.11% and the nitrogen content 0.047%. Thus, the amount of zirconium was insufficient to combine with all the nitrogen as zirconium-nitride. This bar also broke after 6 revolutions of the plastometer when tested at 1000 C.

EXAMPLE III A series of chromium-nickel alloys having chromium contents of 35%, 50%, 60% and 70% were made by vacuum melting the nominal compositions being given in Table I. Each melt was divided into two parts, zirconium being added to one but not to the other. Each part was then cast into an ingot 4 inches long and 1.875 inches in diameter, and was hot-extruded at 1120 C. with an extrusion ratio of 12:1. Test pieces having a gauge length of 0.9 inch and a diameter of 0.1875 inch were machined from each of the extruded rods and tested in the plastometer at temperatures from 800 C. to 1120 C. The results in terms of revolutions to failure are set forth in Table I, and demonstrate that the Zirconium-containing alloys according to the invention (even numbered) have much better ductility over the whole temperature range than the corresponding zirconium-free alloys (odd numbered).

6 EXAMPLE IV A number of ternary type c'hromium-nickel-iron alloys were made, zirconium being added to part of each heat but not to another. The compositions of these alloys and their performance when tested in a torsion plastometer, of course, after being worked into test specimens, are given in Table III.

TABLE I I) Analyzed composition, weight percent Revolutions to failure at 0.)- Alloy Effec- No. Ni Cr C N Z1 tive Zr 800 850 900 1, 000 1, 050 1, 100 1, 120

Balance 35. 5 024 008 Nil 3 3 3 4 10 35. 5 .024 .007 0.69 64 27 45 46 4G 35 35.0 .018 .028 Nil 3 3 3 35. 0 018 028 1.30 1. 12 18 27 32 36 25 4t). 5 011 022 Nil a 3 3 4 8 13 49.5 .011 .022 1.15". 1.01 23 33 38 37 33 49.5 .021 .070 Nil 3 3 3 (1 13 40. 5 021 070 1.85 1. 40 17 31 31 31 27 60.0 021 068 Nil 2 3 5 0 60. 0 021 0G8 l.l5 0. 70 8 15 23 30 21 60. 0 006 060 Nil 2 2 2 5 9. 5 60. 0 006 060 1.80 1. 41 8 14 18 23 23 60. 0 005 110 Nil 1 3 3 7 12 60. 0 005 110 1." 0. 95 3 12 20 2G 18 69. 8 021 053 Nil 2 2 3 16 69. 6 040 053 0.80. 0.45 4 12 21 20 TABLE III Revolutions to failure at Analyzed composition, weight percent C.) A110 Efiec- N0. Cr Ni Fe Zr N tive Z1 950 1,050 1,100 1,120

. 59.5---" Balance. Nil 0.0 1. 5 N.d. 12 N.d. do. 070.. 0. 009 0. 64 31 N.d. 48 N.d. do Nil 0. 0 2 N.d. 10 N.d. 29.5 do 0.75., 0. 009 0.69 39 N.d. 25 N.d. 29.1 do Nil- 0.014 5 N.d. 9 N.d. 29.1 .dO 084 0.014 0.75 32 N.d. N.d. 30.5 do N11 0.008 6 N.d. 10 N.d. 30.5 30.5 do 0 90., 0.008 0.85 22 N.d. 27 N.d. 49.9 Balance 9.7 N1l 0.049 2 N.d. N.d. N.d. 49.9. 9.7-. 100 0. 049 0 68 30 N.d. N.d. N.d. 49.2. 19 4. Nil 0.088 4 9 N.d. 8

l 4 N.d. 7 3 13 N.d. 9 2 5 N.d. 10 4 9 N.d. 8

No'rE: N.d.=Not determined.

Various of the alloys identified in Table I were also subjected to tensile tests in a Hounsfield tensometer at a constant rate of elongation at 1000 C. The results are reported in Table II, and reflect that the capability of the alloys to elongate before fracture is also greatly increased. In fact the alloys with to 70% chromium exhibit the phenomenon known as superplasticity.

TABLE II Tensile tests at an initial rate 010.45 in./in./min. at 1,000 C.

Elongation (percent) Alloy No. (t.s.i.1

1.1 s ps f e'r'r e oaocmoooow NOTE: U.T.S.=U1timate Tensile Strength; t.s.i.=Long tons per Square inch; The elongation tests of Alloys Nos. 10 and 16 were discontinued with the specimen unbroken.

The superior workability of the alloys contemplated in accordance herewith (again even numbered) containing an effective zirconium content is clearly illustrated by the data in Table III. The location of each of these alloys in terms of the chromium, nickel and iron ternary diagram is shown by their respective numbers in the accompanying drawing.

EXAMPLE V Three chromium-nickel alloys were made with additions of cerium, hafnium and yttrium, respectively, the alloy otherwise being produced as described in Example 3. The compositions and the results of plastometer tests on specimens of these alloys are set forth in Table IV. Alloy No. 7, devoid of any eutectic-forming element is also included, for purposes of comparison.

It will be observed that of the three eutectic-forming elements in Table IV hafnium and yttrium were considerably more effective than cerium. At comparable chromium and nitrogen levels zirconium consistently gives superior results than any of the three.

In addition to the foregoing and as more fully described in our copending U.S. patent application Ser. No. 797,679, filed on Feb. 7, 1969, the room temperature ductility of the instant alloys can be improved by special heat treatment. Attempts heretofore have been made to increase room temperature ductility of the base alloys by heat treatment, and specifically wrought 50%-50% chromiumnickel alloys have been heated to a temperature of 1200 C. or above and then water-quenched. Such treatments have not proved satisfactory. However, it has been found that annealing at a lower temperature increases the ductility and lowers the hardness. The annealing temperature is from 600" C. to 850 C., e.g., 650 C. to 750 C. or 800 C. The time required to produce cold-workability depends upon the temperature, broadly being at least 16 hours or longer at 700 C. or at a lower temperature of 600 C. at least to 100 hours and at least 8 hours is suitable at the temperature of 850 C. The temperature may be increased to 1000 0., provided the alloys contain less than about 60% chromium. In this event, the alloys should not be held longer than about /2 hour at this temperature.

It is to be observed that the present invention provides alloys high in chromium content and characterized by improved workability. While the chromium may be as low as 28%, at least 40% should be present in striving for the best corrosion resistance. Alloys containing from 40% or 45% to 70% or 75% chromium, up to about 40% or 45 iron, e.g., up to 30% iron, an effective amount (as described herein) of, most advantageously, zirconium (particularly 0.5% to 1% zirconium) and the balance essentially nickel are quite satisfactory. To less advantage yttrium, hafnium or cerium up to 4.0% can be used in lieu of zirconium. A particularly useful alloy contains nominally 50% chromium, less than 0.1% nitrogen and from 0.5% to 1% zirconium, the balance, except for impurities, being nickel.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. A high chromium, nickel alloy containing a Ni eutectic phase and consisting of from about 28% to 75% chromium, up to 46% iron, at least one element selected from the group consisting of zirconium, yttrium, hafnium and cerium in an effective amount suflicient to form said eutectic phase whereby there is imparted to the alloy improved torsional ductility and enhanced workability, up to about 1% manganese, up to about 1% silicon, and the balance essentially nickel, the chromium, iron and nickel being correlated such that they are represented by a point in or on the lines bounding the area ABCDEA in the accompanying drawing.

2. An alloy in accordance with claim 1 in which the eutectic forming element is zirconium.

3. An alloy in accordance with claim 2 in which the effective amount of zirconium is related and proportional to the chromium content such that when the chromium content is 30% the zirconium percentage is at least 0.2% and when the chromium content is the amount of zirconium is at least 0.35%, the zirconium not exceeding 4%.

4. An alloy in accordance with claim 2 in which the zirconium does not exceed about 2%.

5. An alloy in accordance with claim 1 and containing from 29% to 70% chromium, up to 45% iron, up to 0.1% nitrogen, from 0.5% to 1% zirconium, the balance being essentially nickel.

6. An alloy in accordance with claim 5 containing 40% to 70% chromium and up to 40% iron.

7. An alloy in accordance with claim 6 containing nominally 50% chromium, up to 0.1% nitrogen and the balance, except for impurities, being nickel.

8. An alloy in accordance with claim. 2 which contains from 0.2% to 1% effective zirconium.

9. An alloy in accordance with claim 1 which contains yttrium.

10. An alloy in accordance with claim 1 which contains hafnium.

11. An alloy in accordance with claim 1 which contains cerium.

References Cited UNITED STATES PATENTS 2,157,060 5/ 1939 Schafmeister -12 8 2,238,160 4/1941 Doom 75171 2,809,139 10/1957 Bloom et al 75176 X 3,306,740 2/1967 Wyman et a1. 75176 3,479,157 11/1969 Richards et al 75171 3,519,419 7/1970 Gibson et al 148-32.5 X

FOREIGN PATENTS 793,539 1935 France 75-128 607,975 1935 Germany 75l28 CHARLES N. LOVELL, Primary Examiner US. Cl. X.R. 

